linux-kernel - Re: [PATCH v2] iommu/arm: Add module parameter to set msi iova address

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Message-ID: <20200921204545.GA3811@willie-the-truck>
Date:   Mon, 21 Sep 2020 21:45:45 +0100
From:   Will Deacon <will@...nel.org>
To:     Vennila Megavannan <vemegava@...ux.microsoft.com>
Cc:     robin.murphy@....com, joro@...tes.org, srinath.mannam@...adcom.com,
        jean-philippe@...aro.org, eric.auger@...hat.com,
        shameerali.kolothum.thodi@...wei.com, tyhicks@...ux.microsoft.com,
        linux-arm-kernel@...ts.infradead.org,
        iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        bcm-kernel-feedback-list@...adcom.com
Subject: Re: [PATCH v2] iommu/arm: Add module parameter to set msi iova
 address

On Mon, Sep 14, 2020 at 11:13:07AM -0700, Vennila Megavannan wrote:
> From: Srinath Mannam <srinath.mannam@...adcom.com>
> 
> Add provision to change default value of MSI IOVA base to platform's
> suitable IOVA using module parameter. The present hardcoded MSI IOVA base
> may not be the accessible IOVA ranges of platform.
> 
> If any platform has the limitaion to access default MSI IOVA, then it can
> be changed using "arm-smmu.msi_iova_base=0xa0000000" command line argument.
> 
> Signed-off-by: Srinath Mannam <srinath.mannam@...adcom.com>
> Co-developed-by: Vennila Megavannan <vemegava@...ux.microsoft.com>
> Signed-off-by: Vennila Megavannan <vemegava@...ux.microsoft.com>
> ---
>  drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c | 5 ++++-
>  drivers/iommu/arm/arm-smmu/arm-smmu.c       | 5 ++++-
>  2 files changed, 8 insertions(+), 2 deletions(-)

This feels pretty fragile. Wouldn't it be better to realise that there's
a region conflict with iommu_dma_get_resv_regions() and move the MSI window
accordingly at runtime?

Will